This invention relates to a ground isolation monitoring apparatus for use between an AC power source and an electronic device operated with earth ground disconnected, and more particularly to such apparatus having a protective circuit which automatically reconnects earth ground and interrupts power to the device in the event that exposed conductive portions of the device become energized with hazardous potentials.
It is often desirable to operate certain electronic devices, such as test and measurement instruments, with the protective earth ground disconnected in order to facilitate so-called floating measurements. Floating measurements, i.e., measurements made without reference to ground, are common in situations in which small measurable signals are superimposed on large offsetting voltages which may exceed the input rating of the measuring device. It has been commonplace to insert an isolation transformer between the AC power source and the electronic device to permit floating measurements; however, it is recognized that this practice is often dangerous in terms of both shock hazard and equipment damage. Furthermore, worldwide safety standards have recently been imposed to provide safe working conditions, and therefore it is desired to provide a safe means of facilitating floating measurements.